Do you have the guitar, but not the keyboard, in Rock Band 3, and you are dying to see some of the charts? Fear not: there is a way to play keys on guitar (and vice versa) in Rock Band 3!

Steps

1

Go to "My Goals" in the career menu. Look at the Guitar Immortal goal, which is unlocked by passing 50 songs on Medium guitar (or higher) while getting 5 stars. This will probably not take a day, unless you devote a whole day for playing Rock Band 3.

2

Once you unlock the goal, a screen will come up saying that you can play keys on guitar! Once this happens, it also works vice versa; you can play Guitar and Bass on keys.

3

Pick a song while on the guitar, pick keys, and play to your heart's content!